Eh?

Posted By: Charlie DukeToday a client wanted to use Super 10 UNDER a concrete screed floor to achieve the 1.9 U value "like they told me I could do at Interbuild".

Maybe the force of reflected photons would support the weight of the concrete so the fluffy stuff doesn't get compressed.

A U-value of 1.9W/m^2K sounds about right for a typical thickness MF when it is not used with large air-gaps on either side. Does anybody not agree with this (it is basically the U-value of a still layer of air the same thickness as the MF plus a bit to allow for conduction in the fibres)?

If you install a 30mm MF with, say, 50mm air-gaps either side, then it is not surprising that the U-value is lower and perhaps comparable to 130mm of Cellotex, for example. In the real world, maybe the MF will do better or worse, but this probably has more to do with installation than the fundamental insulation performance. One thing I remain sceptical about is the use of very large air-gaps with MFs. In a double-glazing panel, the optimum gap is less than 25mm for a vertical orientation (because convection starts up in the gap if you make it wider, and that actually reduces its performance). In an inclined panel, such as a roof pitch, the best gap will be wider, but I would be surprised if it can be as wide as 100mm before convection once again becomes an issue. As I have said before, to kill the convection in a wide gap you could introduce something like wool, or better a foam, like Cellotex - but then you don't really need the MF in the middle!

The MF manufacturers seem to be quite careful now with the wording of their product claims. Recently, I have not seen claims like "equivalent to 200mm of Rockwool" without the implication that this only applies when installed with adequate air-gaps (although the specs. can be a bit obtuse about this). However, third parties often lose sight of that condition when they "quote" the manufacturers' data.

would a MF work as a blind/curtain for windows at night, say held in place with some kind of frame ?

Foil curtains look so, oooo, glam.

Well, having just spent the afternoon helping 't OH making some rather large lined curtains I'd baulk at anything more complicated. It was quite hard enough as it was.

I'm not sure that constant opening and shutting of foil cutains might not result in small fragments of foil - it's not designed for it, is it? Anyway, I still can't see that thermal gain from the internal layers justifies the cost.

Well well well. It looks as if multifoil is not dead after all. Actis have just won a high court battle regarding BR443. Here's a snipit from the below link

"Mr Justice Charles, the judge at the hearing, said of the guidance, known as BR 443, that “a fair procedure was not followed in making the revision”.

He ruled that the use of BR 443 to test multifoils would be unenforceable until the proper procedures had been complied with. He also instructed the government to circulate guidance on multifoil insulation and an explanation of the judgment to building control officers."

See http://www.building.co.uk/story.asp?sectioncode=29&storycode=3101308&c=0

Unfortunately, the judge was ruling on a point of law, not on a law of physics.

Posted By: biffvernonUnfortunately, the judge was ruling on a point of law, not on a law of physics.

Which is exactly the point. The point of law seemingly that the technical guidance document BR443 was not prepaired in a fair manner. IE with the input of all those likely to be affected by the outcome. I would ask the question. If multifoil does not work, why would those who make it be excluded from input to such a guidance document? If its as cut and dry as has been suggested, a fair and even test method will show this to be the case. The fact is hot box testing does not reflect real life- the challenge is to devise a methodology which does.

For ease of getting it through BRege etc. I thought I would go for Thinsulex. Both internal and external seem to be approved by the manufacturer. My roofer would prefer me to put it inside but I think external looks the neatest solution. I cant figure out what the tyvec is doing in the structure though (sits just outside the foil) - Do I need to leave gaps in the thinsulex to allow air movement – this seems counter intuitive but if not then the Tyvec seems to be redundant. The reason I have gone for the structure is that it seems to cover all the bases but only if I can make the Thinsulex reasonably air tight.

Sorry if this rather basic question is cutting across the thread a bit – should I have posted else ware?

I can't see why we can't rely on physics. It is perfectly possible to model the behaviour of complex things like nuclear reactors - my son did it as an MSc project. A few layers of foil and some fluffy stuff really should not generate all this angst.

Posted By: biffvernonA few layers of foil and some fluffy stuff really should not generate all this angst.

Who's got the angst?

Posted By: biffvernonIt is perfectly possible to model the behaviour of complex things

That's been done for multifoil - no problem.

Not quite Fostertom. As Mike George said, "I am mystified about the inner layers and no I do not have a theoretical explanation. I do have some ideas about what is happening in the models but nothing I can prove, at least not yet. Unfortunately I only use the software, I have not yet grasped the heavy physics behind it, maybe one day!"

Now do tell me who has really done it.

Posted By: Mark BrinkleyAnyone like to comment on what practical difference this judgement makes?

It looks like Building Control may have to operate on the pre 2006 amendement to Part L, meaning that they may not be able to stop people using multifoil as certified previously. Ie comparitively based test results.

I would also speculate that there are going to be some legal claims for loss of sales.

You've lost me there Saint